1. Find item that remains on floor after attempting to pick up more than once a few different ways.

Either by dragging into player inventory from TAB menu.
Or hovering with player crosshair and selecting "take" or "take into hands".

2. If item remains on floor and does not appear in inventory and rendered the slots in which the item would have been placed into unusable, run away from the vicinity and run back and try again. If same results occur, item is confirmed bugged.

I have however no way of confirming what causes the items or how to spot said items without randomly stumbling upon them while looting.
But I have found two quick fixes that solve the issue, and hopefully shed light into what could be the cause.

FIX 1) Directly drag item into hands via the inventory menu. Item of course will still appear on ground and not in hands. However if attempt to throw like a grenade, the character will proceed with the throw animation and the item will now appear as the projectile and no longer on floor where it was bugged.

FIX 2) Relog back into server and item appears as per normal in inventory.

The fact the bugged item can be thrown like a projectile whilst not appearing in your hands visibly could be a good clue into the many issues with player item in hands, ie: items stuck in hands, not interacting etc. I feel they are all related some how.